Video systems are widely use to record significant events. For example, surveillance systems use video systems to record unusual behavior by individuals or machinery. Most conventional single-camera and multi-camera surveillance systems use video tape recorders (VTR) in the VHS format of video cassette recorders (VCR). Because such recorders can only store a relatively small number of images, about 432,000, they are designed to provide a maximum recording time of about four hours, at thirty frames per second.
However, surveillance systems are intended to record events over essentially an unlimited amount of elapsed time, i.e., twenty-four hours a day, seven days a week, year in, year out. The ability to record events for larger number of hours can be achieved by time-lapse recording of still images. This reduces both the amount of recording media, and review time. In time-lapse recording scenes are recorded at reduced frame rates.
However, large surveillance systems, with hundreds or more cameras, still have a problem with processing and maintaining huge amounts of recording media, and the media is frequently recycled. If too much time has passed, then images desired, perhaps a year later, may have been erased. In addition, tape life is on the order of about twenty cycles, so more frequent cycling results in faster image degradation due to tape wear.
One could, of course record at a much lower frame rate. However, this is undesirable because short term events may be missed, for example, a fleeting glimpse of a face in a crime scene. Current video surveillance systems force the user to select a single compromise frame rate that inherently trades recording time against temporal resolution.
Rather than choosing a single frame rate, a user may prefer to specify a higher frame rate for a recent recording, and a lower frame rate for an older recording, thus optimizing the use of storage media. As frames age, some fraction of the frames can be erased, and the recovered media can be used for a new recording. This technique can be extended to allow many different frame rates, each decreasing with the age of the recording, see U.S. patent application Ser. No. 09/504,721 “Gray Code Data Storage,” filed by Freeman et al., on Feb. 16, 2000.
However, that method depends on two features which are not generally available, a random access memory, and identically sized frames. If the recording media is tape, analog or digital, random access is extremely inefficient. Although hard disk drives allow much faster random access, the additional seek time dramatically lowers the potential frame rate of the recorder. Furthermore, digital videos are often compressed using, for example, MPEG-2, resulting in varying frame sizes depending on distortion and rate parameters. It is possible to record with fixed size compressed frames, but that would be an inefficient use of memory.
Therefore, there is a need for a recording system that can store events recorded over a long period of time, and that can recycle storage using sequential accesses.